We all know Tony was the arrogant one among the Avengers from the very start. He was always about himself, considered himself above all, etc. Cap, on the other hand, is the ideal man, he carries his traits with him from before he became Captain America, humbleness, modesty, leadership and what not.
My question is that in The Avengers, Cap started the argument with Tony starting with "big man in a suit of armour, take that off, what are you?" All Tony did was reply, and it was Cap who stated "Let's go a few rounds!" to which Thor laughs.
Why is Cap angered so easily and why did he lose his temper? He normally is a peace-loving guy who is always against war. Cap turned a normal SHIELD discussion into a Tony Stark criticism.